Forming nanowires

ABSTRACT

A method of forming nanowires, including the forming on a metal region of a layer having through openings, and the forming in the through openings of portions deposited in a chemical bath, forming all or part of the nanowires and extending from the metal region.

The present patent application claims the priority benefit of French patent application FR19/13618 which is herein incorporated by reference.

TECHNICAL BACKGROUND

The present disclosure generally concerns nanostructures, in particular nanowires, and electronic devices using nanowires.

PRIOR ART

Nanowires are defined by elongated structures having in their transverse directions nanometer-range dimensions, that is, dimensions smaller than one micrometer, preferably smaller than 500 nm. Nanowires are used in particular in sensors measuring physical quantities, such as pressure or stress, causing the deformation of the nanowires. In particular, the nanowires are used in pressure sensors, gas sensors, piezoelectric generators, etc. The smaller the dimensions of nanowires and the more numerous they are, the higher the resolution of the sensor and its sensitivity may be.

SUMMARY OF THE INVENTION

There is a need for nanowire manufacturing methods having transverse dimensions smaller than those of nanowires obtained by usual methods.

There is a need for nanowire manufacturing methods enabling to obtain a number of nanowires per surface area unit higher than that obtained by usual methods.

There is a need for manufacturing methods enabling to obtain nanowires more regularly distributed on a surface, in other words periodically, with respect to nanowires obtained by usual methods.

An embodiment overcomes all or part of the disadvantages of known nanowire forming methods.

According to a first aspect, an embodiment provides a nanowire forming method, comprising the forming on a metal region of a layer having through openings, and the forming in the through openings of portions deposited in a chemical bath, forming all or part of the nanowires and extending from the metal region.

According to an embodiment, the chemical bath for forming said portions comprises, in solution:

-   -   a first compound defining a source of metal cations; and     -   a second compound comprising a source of hydroxide, sulfide, or         selenide ions,     -   the concentrations of the first and second compounds and/or         their ratio being lower than a concentration threshold of said         chemical bath, said threshold being such that, when the         concentrations are lower than said threshold, a growth of said         portions parallel to said layer is favored over a growth of said         portions in a thickness direction of said layer, said threshold         being preferably in the order of 10 mM; and/or     -   the chemical bath comprising one or a plurality of additives         adapted to favoring a growth of said portions parallel to said         layer over a growth of said portions in the thickness direction         of said layer, preferably citrate or chloride ions; and/or     -   the first compound being in a superstoichiometric concentration         with respect to the second compound.

According to an embodiment:

-   -   said metal cations are cations of at least one metal from the         group formed of Zn, Cd, Ni, Ag, and Cu; and/or     -   the first compound comprises at least one component from the         group formed of nitrates, acetates, chlorides, and sulfates;         and/or     -   the second compound comprises at least one component from the         group formed of HTMA, of ammonia, of sodium hydroxide, or         thiourea, of selenourea, and of sodium selenite.

According to an embodiment, said portions form first portions of the nanowires, the method comprising the chemical bath deposition of second portions of the nanowires extending from the first portions.

According to an embodiment, the composition of the chemical bath is different for the forming of the first and second portions.

According to an embodiment, the chemical bath for forming the second portions comprises, in solution:

-   -   a first compound defining a source of metal cations; and     -   a second compound comprising a source of hydroxide, sulfide, or         selenide ions,     -   the concentrations of the first and second compounds and/or         their ratio being greater than a concentration threshold of the         chemical bath for forming the second portions, the concentration         threshold of the chemical bath for forming the second portions         being such that, when the concentrations are greater than this         threshold, a growth of the second portions orthogonally to said         layer is favored over a growth of the second portions parallel         to said layer, the concentration threshold of the chemical bath         for forming the second portions being preferably in the order of         20 mM; and/or     -   the chemical bath for forming the second portions comprising one         or a plurality of additives adapted to favoring a growth of the         second portions orthogonally to said layer over a growth of the         second portions parallel to said layer, preferably polyethylene         imide or ethylene diamine; and/or     -   the first compound being in a substoichiometric concentration         with respect to the second compound.

According to an embodiment:

-   -   said metal cations of the chemical bath for forming the second         portions are cations of at least one metal from the group formed         of Zn, Cd, Ni, Ag, and Cu; and/or     -   the first compound of the chemical bath for forming the second         portions comprises at least one component from the group formed         of nitrates, acetates, chlorides, and sulfates; and/or     -   the second compound of the chemical bath for forming the second         portions comprises at least one component from the group formed         of HTMA, of ammonia, of sodium hydroxide, of thiourea, of         selenourea, and of sodium selenite.

According to an embodiment, the method comprises the forming, at an end of the nanowires opposite to said metal region, of an electrically-conductive region in contact with the nanowires.

According to an embodiment, the method comprises the forming of a polymer matrix between the nanowires.

According to an embodiment, the method comprises the removal of said layer.

According to an embodiment, the metal region comprises at least one of the materials of the group formed of gold, of nickel, of copper, of palladium, and of platinum.

According to an embodiment, the metal region has a thickness greater than 100 nm.

According to an embodiment, said layer is obtained by lithography from a layer comprising a block copolymer or from a layer sensitive to electrons or to ultraviolet radiations.

According to an embodiment, said layer is defined by a DNA origami.

According to an embodiment:

-   -   the nanowires have a transverse dimension smaller than 300 nm,         preferably smaller than 50 nm;     -   the nanowires have a length greater than 500 nm, preferably         greater than 1 μm; and/or     -   said layer has a thickness smaller than 100 nm, preferably         smaller than 50 nm.

According to a second aspect, an embodiment provides a nanowire forming method, comprising the forming of a DNA origami having through openings, and the forming in the through openings of portions forming all or part of the nanowires.

According to an embodiment, said portions are deposited in a chemical bath.

According to an embodiment, the origami and the openings that it comprises are formed before the bonding of the origami to a substrate.

According to an embodiment, said portions form first portions of the nanowires, and second portions of the nanowires extending from the first portions are deposited in a chemical bath.

According to an embodiment, the composition of the chemical bath is different for the forming of the first and second portions.

According to an embodiment, the method comprises the forming of a polymer matrix between the nanowires.

According to an embodiment, the method comprises the removal of at least a portion of the DNA origami.

An embodiment provided a device obtained by a method such as defined hereabove, wherein the origami comprises folded DNA strands having portions bound to one another by staples.

According to an embodiment, the DNA strand is located on a layer made of a same material as that of the nanowires, said portions extending from said layer.

According to an embodiment, the DNA origami is located on a metal region and, preferably:

-   -   said metal region has a thickness greater than 100 nm; and/or     -   said portions extend from said metal region.

According to an embodiment, the device comprises, at an end of the nanowires opposite to said metal region, an electrically-conductive region in contact with the nanowires.

According to an embodiment:

-   -   the metal region comprises at least one of the materials from         the group formed of gold, nickel, copper, palladium, and         platinum; and/or     -   the nanowires are piezoelectric, preferably, the nanowires         having a wurtzite-type crystal structure and/or comprise at         least one of the materials from the group formed of zinc oxide,         cadmium sulfide, cadmium selenide, and nickel selenide.

An embodiment provides a device wherein:

-   -   the nanowires have a transverse dimension smaller than 40 nm,         preferably smaller than 20 nm;     -   the nanowires have a length greater than 500 nm, preferably         greater than 1 μm; and/or     -   the nanowires have a density greater than 10 nanowires per         square micrometer, preferably greater than 50 nanowires per         square micrometer; and/or     -   the DNA origami has a thickness in the range from in the order         of 2 nm to in the order of 100 nm, preferably in the order of 10         nm.

An embodiment provides a sensor pixel, comprising a device such as defined hereabove.

An embodiment provides a sensor, preferably of fingerprints, comprising a plurality of pixels such as defined hereabove.

According to an embodiment, the pixels are located on the side of a surface of a substrate comprising, vertically in line with each pixel, at least a portion of a circuit associated with this pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features and advantages, as well as others, will be described in detail in the following description of specific embodiments given by way of illustration and not limitation with reference to the accompanying drawings, in which:

FIG. 1 is a partial simplified cross-section view showing a step of a first embodiment of a nanowire manufacturing method;

FIG. 2 is a partial simplified cross-section view showing another step of the first embodiment;

FIG. 3 is a partial simplified cross-section view showing another step of the first embodiment;

FIG. 4 is a partial simplified cross-section view showing a step of a second embodiment of a nanowire manufacturing method;

FIG. 5 is a partial simplified cross-section view showing another step of the second embodiment;

FIG. 6 is a partial simplified cross-section view showing another step of the second embodiment;

FIG. 7 is a partial simplified cross-section view showing a step of an example of a method of manufacturing a sensor comprising nanowires, implementing the embodiments of FIGS. 1 to 6 ;

FIG. 8 is a partial simplified cross-section view showing another step of the method example; and

FIG. 9 is a partial simplified cross-section view showing another step of the method example.

DESCRIPTION OF THE EMBODIMENTS

Like features have been designated by like references in the various figures. In particular, the structural and/or functional features that are common among the various embodiments may have the same references and may dispose identical structural, dimensional and material properties.

For the sake of clarity, only the steps and elements that are useful for an understanding of the embodiments described herein have been illustrated and described in detail. In particular, steps of forming and/or of removal of various portions of structures are not described in detail, the described embodiments being compatible with usual steps of forming/removal of such portions.

Unless specified otherwise, when reference is made to two elements connected together, this signifies a direct connection without any intermediate elements other than conductors, and when reference is made to two elements coupled together, this signifies that these two elements can be connected or they can be coupled via one or more other elements.

In the following description, when reference is made to terms qualifying absolute positions, such as terms “top”, “bottom”, “left”, “right”, etc., or relative positions, such as terms “above”, “under”, “upper”, “lower”, etc., or to terms qualifying directions, such as terms “horizontal”, “vertical”, etc., unless otherwise indicated, it is referred to the orientation of the drawings.

Unless specified otherwise, the expressions “around”, “approximately”, “substantially” and “in the order of” signify within 10%, and preferably within 5%.

FIGS. 1 to 3 are partial simplified cross-section views showing successive steps of a first implementation mode of a nanowire manufacturing method. More precisely, the nanowires are formed by chemical bath deposition. During such a deposition, a seed surface is placed in contact with a solution defining the chemical bath. The nanowires grow on the seed surface. The material of the nanowires forms from the dissolved content of the solution.

At the step of FIG. 1 , a support 110 is provided. As an example, support 110 is a semiconductor wafer, preferably made of silicon. Preferably, the semiconductor wafer has a front surface (upper surface in the drawings) covered with an insulating layer, not shown.

Support 110 is covered with a metal layer 120. The metal layer thus defines a metal region. As a variant, support 110 is metallic and defines the metal region. The upper surface of metal layer 120, that is, the free surface of the metal region, is intended to form a seed surface on which the future nanowires will be formed in the rest of the method. The thickness of metal layer 120 is preferably greater than or equal to 40 nm or approximately 40 nm, for example, greater than 50 nm, more preferably greater than 100 nm, more preferably still greater than 150 nm. As compared with a thinner layer, this enables to improve the state of the seed surface of layer 120.

Metal region 120 may be made of any metal. However, preferably, the mesh parameter of the metal and the crystal orientation of metal region 120 are adapted to the forming of the crystal lattice of the nanowires. For this purpose, metal region 120 is preferably from the group formed of gold, nickel, copper, palladium, and platinum. More preferably, the metal region is made of gold.

Preferably, before forming metal layer 120, support 110 is covered with a bonding layer, not shown, for example made o chromium or of titanium. The bonding layer enables, as compared with an embodiment where this layer is omitted, to ease the forming of metal layer 120 and avoid various problems of stability and/or of adherence of metal layer 120 on support 110.

One then forms, on the free surface of metal region 120, a layer 130. Layer 130 is a perforated layer, that is, it has, or exhibits, through openings 135.

Openings 135 are preferably arranged in an array, more preferably in a regular array such as an array having, in top view (that is, seen from the upper portion of FIG. 1 ), a symmetry of order two, three, four, or six. Openings 135 are preferably arranged in an array, the array having the same row and column pitches.

Each opening 135 has for example a cross-section shape, that is, a shape in top view, which is rectangular or, preferably, square. The cross-section may also have a rounded shape, for example, circular. Preferably, all openings 135 have the same cross-section shape and, more preferably, the same cross-section dimensions.

Preferably, openings 135 have a transverse dimension A smaller than 300 nm, more preferably smaller than 50 nm, more preferably still smaller than 20 nm. Transverse dimension A is further preferably greater than 5 nm, more preferably greater than approximately 10 nm. Preferably, the array of openings 135 exhibits a distance B between neighboring openings in the range from 0.5 to 3 times the transverse dimension A of openings 135. By distance between two openings, there is meant the distance separating the closest edges of the two openings. In the case of a regular pattern, the pitch of the pattern is defined by value A+B.

Perforated layer 130 may be obtained by electronic lithography from an electron-sensitive layer, for example, a layer of poly(methyl methacrylate), PMMA. Perforated layer 130 may also be obtained by lithography with ultraviolet rays, preferably with so-called deep ultraviolet rays, that is, having wavelengths shorter than 200 nm. The perforated layer then results from a layer sensitive to ultraviolet rays.

Perforated layer 130 may also be obtained from a layer comprising a block copolymer. A block copolymer is defined by an association of at least two immiscible and chemically bonded polymers. Each of the polymers defines a block of the copolymer. The immiscible character results in the forming of separate phases, and one of the phases is then removed to form openings 135. The size of the blocks is then selected to obtain the desired dimensions of openings 135.

Perforated layer 130 may also be replaced with an origami of deoxyribonucleic acid (DNA) such as that of the embodiments described hereafter in relation with FIGS. 4 to 6 .

At the step of FIG. 2 , the material of the nanowires is deposited by a chemical bath. Portions 210 of the deposited material are formed in openings 135 on metal region 120. Portions 210 extend from metal region 120. More particularly, portions 210 are in contact with metal region 120. The deposited material may be any material capable of being formed by chemical bath deposition. Pawar et al.'s publication, entitled “Recent status of chemical bath deposited metal chalcogenide and metal oxide thin films” in Current Applied Physics 11 (2011) 117-161, describes such materials and compositions of chemical baths enabling to deposit these materials. The deposited material may be a metal oxide such as nickel oxide (NiO), silver oxide (AgO), copper oxide (Cu₂O), or for example cadmium oxide (CdO). The deposited material may be a metal hydroxide, preferably iron hydroxide (III) (FeOOH), or copper hydroxide (Cu(OH)₂). The deposited material may also be a chalcogenide, such as cadmium sulfide (CdS), zinc sulfide (ZnS), lead sulfide (PbS), cadmium selenide (CdSe), zinc selenide (ZnSe), or for example, nickel selenide (NiSe). Preferably, the deposited material has piezoelectric properties, more preferably, the deposited material is zinc oxide (ZnO).

Preferably, the temperature of the chemical bath is in the range from 60° C. to 100° C. The deposition is preferably performed for a duration in the range from 1 minute to 60 minutes, more preferably from 5 minutes to 30 minutes.

Although the shown structure has its layer 130 facing the top of the drawing, the deposition is preferably performed with layer 130 facing downwards. This enables to protect portions 210 against possible particles of the material that might precipitate in the chemical bath. The deposition is preferably performed in the absence of an electric field in the solution, which enables to simplify the deposition method.

According to an embodiment, each portion 210 forms a nanowire. Preferably, portions 210 entirely fill openings 135, and the length of the nanowires is then equal to the thickness of perforated layer 130. According to another embodiment, described hereafter in relation with FIG. 3 , each nanowire is formed by chemical bath deposition on one of portions 210.

Thus, the number and the positions of the nanowires correspond to the number and to the positions of portions 210. Now, perforated layer 130 enables to obtain a number of portions 210 greater than the number of portions which would be obtained by omitting perforated layer 130. The perforated layer thus enables to increase the number of nanowires per surface area unit.

Further, due to the fact that the positions of the obtained portions 210 correspond to those of openings 135, perforated layer 130 enables to obtain portions 210 more regularly distributed than portions which would be obtained without perforated layer 130. Perforated layer 130 thus enables to increase the regularity of the positions of the nanowires.

The transverse dimensions of the nanowires are the transverse dimensions of portions 210 (that is, lateral dimensions in the orientation of the drawing), or are a function of the transverse or lateral dimensions of portions 210. Further, each portion 210 has lateral dimensions smaller than, preferably equal to, the transverse dimensions of openings 135. Thus, perforated layer 130 enables to obtain the desired dimensions of the nanowires more easily than in the absence of a perforated layer.

In the preferred case of forming of ZnO nanowires on a gold metal layer 120, the chemical bath for forming portions 210 preferably comprises, in solution:

-   -   zinc nitrate, Zn(NO₃)₂, and Hexamethylenetetramine, HMTA, in         concentrations smaller than 10 mmol/L; and/or     -   Zn(NO₃)₂ in a superstoichiometric concentration with respect to         an HMTA concentration; and/or     -   one or a plurality of additives adapted to favoring a transverse         growth (parallel to layer 130) of portions 210, over a growth of         portions 210 in the thickness direction of layer 130. This or         these additive(s) preferably comprise citrate or chloride ions.

The inventors have observed that the above-defined compositions of the chemical bath, particularly a concentration smaller than the threshold of 10 mmol/L (unit often noted mM) of Zn(NO₃)₂ and of HTMA, enable to guarantee that a portion 210 is formed in each of openings 135. These compositions further enable to ascertain that, in each opening 135, portion 210 entirely covers the bottom of opening 135. In other words, the number of portions 210 is equal to that of openings 135 and the transverse dimensions of each portion 210 may be equal to those of openings 135. The nanowires resulting from such a chemical bath are thus more regularly distributed and/or have more regular transverse dimensions than for chemical baths having different compositions.

Based on the compositions defined hereabove to form ZnO nanowires on a gold metal region 120, those skilled in the art are capable of defining, by routine tests, Zn(NO₃)₂ and HMTA concentration thresholds for other metals of the metal region.

Those skilled in the art are further capable of defining chemical bath compositions adapted to the deposition of other materials than ZnO, such as those defined hereabove, or the materials of the nanowires of the example of a sensor described hereafter in relation with FIGS. 7 to 9 . In particular, Zn(NO₃)₂ and HMTA form, in the case of the forming of ZnO nanowires, first and second respective compounds capable of being, in the case of the forming of other materials, different from Zn(NO₃)₂ and/or HMTA.

The first compound has, when it is in solution in the chemical bath, cations of at least one metal used to form the formed nanowires. In other words, the first compound forms a source of metal cations. This or these metal(s) are preferably from the group formed of zinc (Zn), cadmium (Cd), nickel (Ni), silver (Ag), and copper (Cu). As an example, the first compound comprises one or a plurality of components among nitrate, acetate, chloride, or for example, sulfate, of the considered metal(s). The first compound may thus comprise or be formed of one or a plurality of components among zinc nitrate (Zn(NO₃)₂), zinc acetate (Zn(CH₃COO)₂), zinc chloride (ZnCl₂), zinc sulfate (ZnSO₄), and more generally components in form M(NO₃)₂, MNO₃, M(CH₃COO)₂, M(CH₃COO), MCl₂, MCl, MSO₄, where M is a metal preferably from the above-described list.

In the case where the formed material is a metal oxide, the second compound comprises a source of hydroxide ions, OH⁻. As an example, the second compound may comprise, preferably be made of, an amine, more particularly hexamethylenetetramine (HTMA) and/or ammoniac (NH₃), and/or sodium hydroxide (NaOH). The second compound may thus enable to adjust the pH of the solution to a value adapted to the deposition. In the case where the formed material is a metal sulfide, the second compound preferably comprises a sulfide source, for example, comprises thiourea (CS(NH₂)₂) or sodium sulfide (Na₂S). In the case where the formed material is a metal selenide, the second compound preferably comprises a selenide source, for example, comprises selenourea (CSe(NH₂)₂), or sodium selenite (Na₂SeSO₃).

Those skilled in the art are then capable of defining, by routine tests, a concentration threshold so that, when the concentrations of the first and second compounds are lower than this threshold, the transverse growth of portions 210 is favored, for example, over a growth of portions 210 in the thickness direction of layer 130. This threshold may have a value in the order of 10 mM, for example, equal to 10 mM. It is here considered that the concentrations of the first and second compounds correspond to their concentrations at the time of their introduction into the growth bath. The provision of concentrations lower than the above-defined concentration threshold enables to form a portion 210 in each of openings 135.

The step of FIG. 3 is implemented when, at the end of the step of FIG. 2 , each portion 210 only forms a first portion of a future nanowire. Second portions 310 of the nanowires are then deposited in a chemical bath. Portions 310 extend from portions 210 away from layer 130. The assembly of a portion 210 and of portion 310, formed on portion 210, forms a nanowire 320.

Preferably, the temperature of the chemical bath is in the range from 60° C. to 100° C. The deposition is preferably performed for a duration in the range from 1 minute to 180 minutes according to the length of the nanowires which is desired to be obtained. Nanowires 320 have a length greater than 500 nm, preferably greater than 1 μm.

The step of FIG. 3 thus enables to obtain nanowires 320 having a length greater than the thickness of perforated layer 130. Preferably, perforated layer 130 then has a thickness smaller than 100 nm, preferably smaller than 50 nm. As compared with a thicker perforated layer 130, this enables to accelerate the step of FIG. 2 and enables to increase the free length of nanowires 320. By free length, there is meant a length over which nanowires 320 are not surrounded with a solid material. The larger the free length, the more easily deformable the nanowires, which advantageously increases the sensitivity of a sensor using nanowire deformations.

An advantage of the step of FIG. 3 is that nanowires 320 may have a form factor, defined by the ratio of the length of the nanowires to the smallest transverse dimensions of the nanowires, greater than the form factor of nanowires formed of portions 210 only.

The composition of the chemical bath used to form portions 310 is different from that of the chemical bath used to form portions 210. Thus, in the example of forming of ZnO nanowires, the chemical bath comprises, in solution: Zn(NO₃)₂ and HMTA, in concentrations greater than 20 mM; and/or

-   -   Zn(NO₃)₂ in a substoichiometric concentration with respect to an         HTMA concentration; and/or     -   one or a plurality of additives adapted to favoring a growth of         portions 310 orthogonally to layer 130 over a transverse growth         of portions 310. Such an additive may comprise a polyethylene         imine PEI, or ethylene diamine.

The above-described composition of the chemical bath enables to form, from portions 210, portions 310 extending vertically, that is, orthogonally to the surface of metal region 120, in other words orthogonally to layer 130. Portions 310 having transverse dimensions substantially equal to the transverse dimensions of portions 210 can thus be obtained. This enables to obtain nanowires having a substantially constant cross-section along substantially the entire length of each nanowire. In other words, the nanowires are substantially cylindrical, with an axis of revolution or not, or substantially prismatic. As compared with nanowires having non-constant cross-sections, nanowires with a constant cross-section enable to improve the operation of a device using these nanowires with a constant cross-section.

In the same way as for the chemical bath described in relation with FIG. 2 , the chemical bath described in relation with FIG. 3 may be adapted to other deposited materials than ZnO. For this purpose, Zn(NO₃)₂ and HMTA respectively form the first and second above-described compounds, which may be different from Zn(NO₃)₂ and HMTA. In particular, those skilled in the art are capable of determining, by routine tests: a concentration threshold so that, when the concentrations of the first and second compounds are lower than this threshold, the transverse growth of portions 310 orthogonally to layer 130 is favored over a transverse growth of portions 310. This threshold may have a value in the order of 20 mM, for example, equal to 20 mM; and/or additives also enabling to favor the growth of portions 310 orthogonally to layer 130 over a transverse growth of portions 310.

FIGS. 4 to 6 are partial simplified cross-section views showing successive steps of a second embodiment of a nanowire manufacturing method.

At the step of FIG. 4 , a support 110 and a metal region 120, identical or similar to those described in relation with FIG. 1 and arranged identically or similarly, are provided.

According to the shown embodiment, a seed layer 410 is formed on metal region 120. The future nanowires will be formed on top of and in contact with the free surface of seed layer 410. Preferably, seed layer 410 comprises, for example, is made of, the same material as that of the future nanowires. Preferably, seed layer 410 is made of ZnO. More preferably, the seed layer is a layer of ZnO nanoparticles. By nanoparticles, there is meant particles having their largest dimensions smaller than one micrometer, preferably smaller than 500 nm. In a variant (not shown), metal region 120 is omitted. In another variant (not shown), layer 410 and metal region 120 are omitted and the upper surface of support 110 defines a seed surface.

According to another embodiment, layer 410 is omitted and the surface of metal region 120 forms a seed surface, as described in relation with FIGS. 1 to 3 .

There is formed on the seed surface an origami of deoxyribonucleic acid DNA 430. By DNA origami, there is meant a three-dimensional structure formed by an assembly of folded DNA strands. Portions of the different DNA strands are bound to one another by staples. The staples are preferably pieces of DNA. The bases of DNA strands are selected, for example, by means of a current software, so that the folding of the DNA strands in aqueous solution in the presence of the staples forms the desired three-dimensional structure.

DNA origami 430 is in contact with the seed surface and covers all or part of the seed surface. Thus, DNA origami 430 is preferably located on layers 120 and 410. In the case where layer 410 is omitted and metal layer 420 is made of gold and defines the seed layer, thiol groups may be provided to bind the DNA origami 430 to the seed surface.

The DNA origami 430 exhibits through openings 435. DNA origami 430 has an average thickness C, defined outside of the openings. Average thickness C is preferably uniform over the seed surface or over the portion of the seed surface covered with the DNA origami. Thus, the DNA origami has the three-dimensional structure of a perforated layer. Preferably, average thickness C is in the range from a few nanometers, that is, from in the order of 2 nm to 10 nm, to a few tens of nanometers, that is, from in the order of 20 nm to 100 nm. More preferably, average thickness C is in the order of 10 nm, for example, equal to nm.

Openings 435 are preferable arranged in an array, more preferably in a regular array such as an array having, in top view, a symmetry of order two, three, four, or six. Openings 435 are preferably arranged in an array, the array having the same row and column pitches. As an example, the row and column pitch is in the range from 15 nm to 30 nm, preferably equal to 20 nm or to 25 nm.

Each opening 435 for example has a rectangular or, preferably, square, cross-section shape. The cross-section of each opening 435 may also have a rounded shape, for example, substantially circular. Preferably, all openings 435 have the same cross-section shape and the same cross-section dimensions.

Openings 435 for example have a transverse dimension A1 smaller than 100 nm, preferably smaller than 40 nm, more preferably smaller than 20 nm, more preferably still smaller than 15 nm. Transverse dimension A1 is further preferably greater than 5 nm. More preferably, transverse dimension A1 is in the order of 10 nm. Preferably, the array of openings 435 has a distance B1 between neighboring openings in the range from 0.5 to 3 times the transverse dimensions A1 of openings 435.

At the step of FIG. 5 , openings 435 are filled with the material of the nanowires. Portions 210 of the material are thus formed in openings 435. Portions 210 extend from the seed surface. More precisely, portions 210 are in contact with seed layer 410 or, if the latter is omitted, with metal layer 120. The material of portions 210 may be any material capable of being formed by chemical bath deposition, such as described in relation with FIG. 2 . Thus, the material of portions 210 may be a metal oxide such as NiO, AgO, Cu₂O, or for example, CdO. The deposition material may be a metal hydroxide, preferably FeOOH or Cu(OH)₂. The deposited material may also be a chalcogenide, CdS, ZnS, PbS, CdSe, ZnSe, or for example NiSe. Preferably, the deposited material has piezoelectric properties, more preferably, the material of portions 210 is zinc oxide (ZnO).

Preferably, portions 210 are formed by chemical bath deposition, as described in relation with FIG. 2 . This is not limiting, and portions 210 may be formed by any usual method enabling to form portions in through openings of a perforated layer. As an example, the deposition may be performed in the presence or in the absence of an electric field, or by electrolysis. The deposition may also be performed over the entire surface of the structure obtained at the step of FIG. 4 , the portions located above the upper level of DNA origami 430 being then possibly removed, partly or totally, for example, by polishing.

Preferably, at the step of FIG. 6 , the DNA origami (430, FIG. 5 ) is removed. This removal is for example performed by a plasma adapted to selectively etching the DNA origami over the material of portions 210.

According to an embodiment, each portion 210 forms a nanowire. Preferably, portions 210 entirely fill openings (435, FIG. 5 ), and the length of the nanowires is then equal to thickness C (FIG. 4 ) of the DNA origami (430, FIG. 5 ).

According to another embodiment, each nanowire is formed by chemical bath deposition on one of portions 210, as described hereabove in relation with FIG. 3 . The nanowires then preferably have a length greater than 500 nm, more preferably greater than 1 μm. The step of FIG. 6 may then be omitted.

It could as a variant be devised to replace the DNA origami with a perforated layer such as the layer 130 of the method of FIGS. 1 to 3 , this perforated layer being obtained by electron lithography, by ultraviolet radiation, or from a block copolymer. However, as compared with such as variant, the DNA origami enables to obtain nanowires of smaller diameter and/or to reach a greater density of nanowires on the surface. As an example, the diameter of the nanowires may be smaller than 10 nm. Further, the nanowire density is preferably greater than 10 nanowires per μm², for example, greater than 50 nanowires per μm², preferably equal to, or greater than, 625 per μm², or even equal to, or greater than, 1,600 per μm². The DNA origami thus enables to improve the accuracy and the sensitivity of a sensor using the obtained nanowires.

FIGS. 7 to 9 are partial simplified cross-section views, showing steps of an example of a method of manufacturing a sensor comprising nanowires. This method implements the steps of FIGS. 1 to 3 or the steps of FIGS. 4 to 6 . The sensor is for example a fingerprint sensor. In particular, the sensor comprises a pixel array. A single pixel has been shown, the other pixels being similar or identical to the shown pixel.

At the step of FIG. 7 , a support 110 formed by a semiconductor substrate, for example, made of silicon, is provided. Preferably, for each pixel, the sensor comprises a pixel control/read circuit, not shown, comprising transistors, for example, MOS-type transistors. The circuit is preferably of CMOS type.

Preferably, all or part of the transistors of the circuit associated with each pixel are located inside and on top of substrate 110, vertically in line with a location 710 where the nanowires will be formed. The location 710 of each pixel typically has a square shape in top view. For example, the dimensions of sides of location 710 are in the range from 0.8 μm to 1.5 μm, preferably are of approximately 1 μm, more preferably are equal to 1 μm.

Each pixel comprises two electrically-conductive regions 720 and 722 located on the front surface side of substrate 110 (that is, in the upper portion of the substrate). Electrically-conductive regions 722 are electrically connected to the circuit associated with the pixel.

The front surface of substrate 110 is covered with an electrically-insulating layer 730, typically made of silicon oxide. Insulating layer 730 is thoroughly crossed by a conductive via 732 located on conductive region 722.

A metal layer 740 covering insulating layer 730 is then formed. Metal layer 740 forms a metal conductive region. Conductive via 732 places metal layer 740 in electric contact with conductive region 722. An opening 742 is provided in metal layer 740 vertically in line with, that is, in front of, conductive region 720. Metal layer 740 is identical or similar to the metal layer 120 of the embodiments described in relation with FIGS. 1 to 6 .

One forms, on metal layer 740, nanowires 750 as described hereabove in relation with FIGS. 1 to 3 and/or with FIGS. 4 to 6 to form nanowires 210 or 320. Preferably, nanowires 750 are in contact with metal layer 740.

Preferably, nanowires 750 are formed only at location 710. For this purpose, in the case where nanowires 750 are formed in an aqueous solution such as a chemical bath, the surface of metal layer 740 may be made hydrophobic outside of location 710.

At the step of FIG. 8 , a polymer layer 810 filling the space between nanowires 750 is formed. Layer 810 thus forms an electrically-insulating polymer matrix. More precisely, the polymer is more flexible than the material of nanowires 750, that is, it has a modulus of elasticity smaller, for example, more than 10 times smaller, that that of nanowires 750. Preferably, layer 810 is formed so that the upper end of nanowires 750, that is, the end of nanowires 750 opposite to metal layer 740, is flush with the upper surface of layer 810. Preferably, layer 810 covers the front surface of the structure obtained at the step of FIG. 7 outside of location 710. Layer 810 is in contact with insulating layer 730 in the opening 742 of metal layer 740.

At the step of FIG. 9 , a conductive via 910 thoroughly crossing layer 810 and located vertically in line with conductive region 720 is formed. Conductive via 910 runs through the opening 742 of conductive layer 740. Conductive via 910 is insulated from the lateral walls of opening 742 by portions of layer 810.

An electrically-conductive region 920 covering nanowires 750 is also formed. Conductive region 920 is in contact with the upper ends of nanowires 750. Conductive region 920 may be made of the same material as via 910. Regions 920 and the material of via 910 may then be simultaneously formed. Conductive region 920 may also be formed after the material of via 910, and conductive region 920 and conductive via 910 may then be made of different materials. In a variant, the material of conductive layer 920 is transparent in a wavelength range. As an example, the wavelength range corresponds to visible radiations, that is, in the range from approximately 400 nm to approximately 800 nm. By transparent layer, there is meant that more than 50%, preferably more than 90%, of any radiation in the wavelength range entering the layer perpendicularly through one of the main surfaces of the layer (parallel to the plane of the layer) comes out of the layer through the other one of the main surfaces. In this variant, layer 810 is also transparent. When the sensor is submitted to a radiation crossing conductive layer 920, this radiation can thus be detected due to its interaction with the nanowires.

Preferably, the conductive regions 920 of neighboring pixels are insulated from one another. To achieve this, conductive regions 920 are preferably obtained by steps of:

-   -   forming a conductive layer comprising the future conductive         regions 920;     -   covering this conductive layer with a lithographed layer, not         shown, exhibiting openings outside of the locations of         conductive regions 920; and     -   etching the portions of the conductive layer located vertically         in line with the openings of the lithographed layer.

In each pixel thus obtained, nanowires 750 are arranged parallel to one another and have their end in respective contact with conductive regions 740 and 920. Conductive regions 740 and 920 are in electric contact with respective regions 722 and 720 via respective vias 732 and 910. Regions 722 and 720 form the electrodes of the pixel.

The fact for substrate 110 to comprise, vertically in line with each pixel, at least a portion of the circuit associated with this pixel, enables, with respect to a sensor where the circuits are not vertically in line with the pixels, to increase the compactness and/or the resolution of the sensor. Further, the fact for the nanowires to have transverse dimensions such as defined hereabove enables, as compared with nanowires having larger lateral dimensions, to decrease the pixel size and thus increase the sensor resolution. Thus, the resolution of the obtained sensor may be smaller than 50 μm, preferably in the order of 1 μm.

Preferably, the nanowires are piezoelectric, that is, are made of a piezoelectric material. In operation, a pressure or a force exerted on region 920, for example, towards the bottom of FIG. 9 , deforms the nanowires and causes a potential difference measured by the circuit associated with the pixel. For this purpose, as an example, the material of the nanowires is selected from among zinc oxide, ZnO, cadmium sulfide, CdS, and cadmium selenide, CdSe. The piezoelectric material may also be selected from among materials having a wurtzite-type crystal structure. The nanowires may also comprise a plurality of these materials.

It is preferred for metal region 740 to be directly in contact with nanowires 750. A Schottky-type electric contact is thus preferably formed between metal region 740 and nanowires 750. The sensitivity of the sensor is then advantageously greater than that of a similar sensor but further comprising a layer such as layer 410 (FIGS. 4 to 6 ) located between metal region 740 and nanowires 750.

Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these embodiments can be combined and other variants will readily occur to those skilled in the art.

In particular, although embodiments where the forming of origami 430 on layer 120 or 740 is performed while this layer is supported by support 110 has been more particularly been described, it may be provided, according to another embodiment, to form origami 430 on layer 120 before transferring it onto support or substrate 110.

Finally, the practical implementation of the described embodiments and variations is within the abilities of those skilled in the art based on the functional indications given hereabove. 

1. Method of forming nanowires, comprising: the forming on a metal region of a layer having through openings; the forming in the through openings of portions deposited in a first chemical bath, forming first portions of the nanowires and extending from the metal region; and the deposition in a second chemical bath, of a composition different from that of the first chemical bath, of second portions of the nanowires extending from the first portions.
 2. Method according to claim 1, wherein the first chemical bath for forming said portions comprises, in solution: a first compound defining a source of metal cations; and a second compound comprising a source of hydroxide, sulfide, or selenide ions, the concentrations of the first and second compounds and/or their ratio being lower than a concentration threshold of said first chemical bath, said threshold being such that, when the concentrations are lower than said threshold, a growth of said portions parallel to said layer is favored over a growth of said portions in a thickness direction of said layer, said threshold being preferably in the order of 10 mM; and/or the first chemical bath comprising one or a plurality of additives adapted to favoring a growth of said portions parallel to said layer over a growth of said portions in the thickness direction of said layer, preferably citrate or chloride ions; and/or the first compound being in a superstoichiometric concentration with respect to the second compound.
 3. Method according to claim 2, wherein: said metal cations are cations of at least one metal from the group formed of Zn, Cd, Ni, Ag, and Cu; and/or the first compound comprises at least one component from the group formed of nitrates, acetates, chlorides, and sulfates; and/or the second compound comprises at least one component from the group formed of HTMA, of ammonia, of sodium hydroxide, of thiourea, of selenourea, and of sodium selenite.
 4. Method according to any of claim 1, wherein the second chemical bath for forming the second portions comprises, in solution: a first compound defining a source of metal cations; and a second compound comprising a source of hydroxide, sulfide, or selenide ions, the concentrations of the first and second compounds and/or their ratio being greater than a concentration threshold of the second chemical bath for forming the second portions, the concentration threshold of the second chemical bath for forming the second portions being such that, when the concentrations are greater than this threshold, a growth of the second portions orthogonally to said layer is favored over a growth of the second portions parallel to said layer, the concentration threshold of the second chemical bath for forming the second portions being preferably in the order of 20 mM; and/or the second chemical bath for forming the second portions comprising one or a plurality of additives adapted to favoring a growth of the second portions orthogonally to said layer over a growth of the second portions parallel to said layer, preferably polyethylene imide or ethylene diamine; and/or the first compound being in a substoichiometric concentration with respect to the second compound.
 5. Method according to claim 4, wherein: said metal cations of the second chemical bath for forming the second portions are cations of at least one metal from the group formed of Zn, Cd, Ni, Ag, and Cu; and/or the first compound of the second chemical bath for forming the second portions comprises at least one component from the group formed of nitrates, acetates, chlorides, and sulfates; and/or the second compound of the second chemical bath for forming the second portions comprises at least one component from the group formed of HTMA, of ammonia, of sodium hydroxide, of thiourea, of selenourea, and of sodium selenite.
 6. Method according to claim 1, comprising the forming, at one end of the nanowires opposite to said metal region, of an electrically-conductive region in contact with the nanowires.
 7. Method according to claim 1, comprising the forming of a polymer matrix between the nanowires.
 8. Method according to claim 1, comprising the removal of said layer.
 9. Method according to claim 1, wherein the metal region comprises at least one of the materials from the group formed of gold, of nickel, of copper, of palladium, and of platinum.
 10. Method according to claim 1, wherein the metal region has a thickness greater than 100 nm.
 11. Method according to claim 1, wherein said layer is obtained by lithography from a layer comprising a block copolymer or from a layer sensitive to electrons or to ultraviolet radiations.
 12. Method according to claim 1, wherein said layer is defined by a DNA origami.
 13. Method according to claim 1, wherein: the nanowires have a transverse dimension smaller than 300 nm, preferably smaller than 50 nm; the nanowires have a length greater than 500 nm, preferably greater than 1 μm; and/or said layer has a thickness smaller than 100 nm, preferably smaller than 50 nm. 